Submerge Question

I get that troops Avoid AoE when they are submerged, but how exactly is that handled when it comes to “damage randomly split between troops” spells?

Eg:
My Opponent has 3 of his 4 troops submerged.
I cast a Spell that does 40 damage randomly split between troops.
Now obviously 3 of the troops are going to take 0 damage but what about the 4th?
Will he take the full 40 damage, or has a portion of that been wasted missing the submerged troops?

Another way to put it, Which of these is correct?:
a) Since three troops are submerged (and cant be seen) the random damage is split between the only visible troops (1)
b) Damage is split between all 4 troops just as it would be if no one was submerged and damage is “wasted” on the water the troops are submerged under.

I think it’s randomly split still. Makes sense.

Damage that “randomly splits amongst enemies” actually targets all enemies which is why submerge will protect against it.

But one single non submerged troop won’t get the full damage, just a percentage of it?